Epoxy compositions are in wide-spread commercial use as adhesives, providing very strong bonding between surfaces by virtue of the three-dimensional cross-linked polymeric network that is formed upon curing of the epoxy. Curable epoxy compositions nearly universally comprise a plurality of polymeric molecules each having a plurality of cross-linkable epoxide functional groups, and optionally, one or more so-called curing agents, or cross-linking agents. Epoxy can form cross linking by itself without curing agent or catalyst, albeit slowly. Curing agents contain functional groups that can react with epoxy groups, provide the desired cross-links, expedite the cross-linking process, and control cross-link density. Catalyst is added to accelerate the cure. A latent catalyst is one that is typically thermally activated, and serves to induce a high rate of curing at a controlled temperature.
Curable epoxy compositions normally comprise curing catalysts in order to reduce the curing time thereof. However, the presence of a curing catalyst in a curable composition in storage tends to induce some degree of undesirable premature curing thereby reducing storage time. Refrigeration is often used to prevent untimely curing of a stored uncured or partially cured epoxy composition. Cure rate can be increased at higher catalyst loading, but generally at the expense of storage life. This balance between need for storage life and cure rate limits the amount of catalyst that can be employed. A latent or in-situ generated catalyst allows a shift in balance towards higher reaction rate without sacrificing storage life.
In order to bring the above stated problems under control, it has been found in the art to employ so-called latent catalysts. A latent catalyst is one that does not catalyze curing of the epoxy composition until some threshold condition, typically a threshold temperature, is reached. However, the requirements for a latent catalyst to be of practical use are stringent. A latent catalyst should not be activated until the desired curing temperature is reached, typically in the range of 120 to 200° C.
Most latent catalysts exist in the same chemical form under both storage and curing conditions. It is desirable to have an in-situ generated catalyst which exists in more inert form during storage conditions but which generates active chemical species during cure, thereby achieving fast cure rate without sacrificing storage life of the epoxy composition. If the catalyst exists in the solid state during storage conditions, reactivity is also lowered due to reduced mobility.
Many candidate latent catalysts are solids that are insoluble in many commonly used organic solvents requiring intensive dispersion in the epoxy compound to achieve homogeneity. Tertiary amines such as benzyldimethylamine are liquids, but catalytic effectiveness is limited. Quaternary ammonium halides are known but halide ions are undesirable contaminants in many applications. Quaternary ammonium acetates exhibit excessive moisture absorption.
Phosphonium and alkali metal bicarbonate compositions are employed as components in epoxy compositions in order to effect chain extension of the epoxy molecules.
Ikeda et al., Japanese Unexamined Patent Application Publication 03149216, discloses a photocurable epoxy composition comprising an onium bicarbonate. A second aluminum organic compound activatable by UV-visible radiation is also required. Exemplified is Ph2SHCO3 which undergoes photolysis to form H2CO3.
Bertram et al. U.S. Pat. No. 4,405,766, discloses the use of tetrahydrocarbyl phosphonium bicarbonate as curing catalysts for epoxy resins containing dicarboxylic acid or anhydride crosslinking agents.
Doorakian et al., U.S. Pat. No. 4,354,015, discloses tetrahydrocarbyl phosphonium bicarbonate salts as catalysts for promoting the reaction between (a) vicinal epoxides and (b) phenols. These catalysts are particularly useful in preparing high molecular weight epoxy resins by the advancement reaction of an epoxy resin with a polyhydric phenol. The phosphoium bicarbonates are said by Doorakian et al. to be surprisingly effective at producing substantially linear reaction products.
Dershem, U.S. Patent Application Publication US20100056671, discloses catalysts that can be employed to catalyze the reaction between a phenolic hydroxyl group, a phenyl ester, an anhydride, or a thiol and a vicinal epoxide. Disclosed are quaternary ammonium compounds including benzyl trimethyl ammonium chloride and tetrabutylammonium chloride, as well as butyltriphenylphosphonium bicarbonate. US
Christiansen et al., U.S. Pat. No. 7,592,067 discloses alkali-metal-containing cure accelerators for epoxy-coated articles, including an alkali-metal-containing bicarbonate.